Although the N-nitration by nitric acid is widely used to synthesize nitramines in biological, medical, and explosive industries, little is known about the microscopic behavior when the nitrated substrates are tertiary amines. Hexahydro-1,3,5-triacetyl-s-triazine (TRAT) nitrated into hexahydro-1,3,5-trinitro-s-triazine (RDX) was theoretically investigated at the MP2/cc-PVDZ level. An O-to-N-transnitration mechanism was put forward for the N-nitration of N-acetyl tertiary amines, including the formation of diverse complexes R'N(COCH3)RNO2+ and deacetylate. The electron transfer results in the complex formation, and the acetyl-to-nitro electrophilic displacement leads to deacetylate. Presumably, the carbonyl groups (C=O) in N-acetyl tertiary amines serve as the hinged joint in the electron transfer. Three successive N-nitrations transform TRAT into RDX; their electron transfers are strongly exothermic by 21.1, 19.5, and 15.4 kcal/mol relative to TRAT + 3NO(2)(+), repectively, and their electrophilic displacements possess low activation Gibbs free energies of 9.0, 6.8, and 7.5 kcal/mol relative to the a-complexes 6, 11, and 14, respectively. The rate constants of the single electron transfer (SET) and the acetyl-to-nitro displacement were estimated roughly by Marcus and transition-state (TS) theories, respectively, indicating that they are both fast with the strong exothermicity. The available experimental phenomena were well interpreted by the computational results.